Horn-reflector antennas have been known for many years. For example, a 1963 article in The Bell System Technical Journal describes a conical horn-reflector antenna for use in satellite communication ground stations (Hines et al., "The Electrical Characteristics Of The Conical Horn-Reflector Antenna", The Bell System Technical Journal, July 1963, pp. 1187-1211). A conical horn-reflector antenna is also described in Dawson U.S. Pat. No. 3,550,142, issued Dec. 22, 1970. A 1969 article by Y. Takeichi et al. entitled "The Diagonal Horn-Reflector Antenna", IEEE G-AP Symp., pp. 279-285, Dec. 9-11, 1969, describes a so-called "diagonal" horn-reflector antenna, in which the flared horn has a square aperture (i.e., the cross section of the horn, taken in a plane perpendicular to its axis, is square).
One of the problems with horn-reflector antennas is that loose materials can enter the feed horn after the antenna has been installed, particularly when the antenna remains in the field over a period of years and is subjected to varying environmental conditions. For example, it is not uncommon for bullets to enter such antennas as a result of vandalism, and various other types of particulate matter also enter such antennas from time to time. The absorber material that is used to line certain portions of such antennas can also fracture or become dislodged. Any of these loose materials which enter or break loose in the interior of the antenna fall down through the tapered feed horn and either accumulate in bends in the waveguide or become trapped in the feed horn or the waveguide. These accumulations of particulate material and other debris tend to lodge in close to the waveguide entry and can seriously degrade the antenna performance, such as by increasing the attenuation of the microwave signals transmitted and/or received by the antenna.